Olefins are traditionally produced from petroleum feedstocks by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstocks. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefin(s). The preferred alcohol for light olefin production is methanol and the preferred process for converting a methanol-containing feedstock into light olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a molecular sieve catalyst composition.
There are many different types of molecular sieves well known to convert a feedstock, especially an oxygenate containing feedstock, into one or more olefin(s). For example, U.S. Pat. No. 5,367,100 describes the use of the well known zeolite, ZSM-5, to convert methanol into olefin(s); U.S. Pat. No. 4,062,905 discusses the conversion of methanol and other oxygenates to ethylene and propylene using crystalline aluminosilicate zeolites, for example Zeolite T, ZK5, erionite and chabazite; U.S. Pat. No. 4,079,095 describes the use of ZSM-34 to convert methanol to hydrocarbon products such as ethylene and propylene; and U.S. Pat. No. 4,310,440 describes producing light olefin(s) from an alcohol using a crystalline aluminophosphate.
One of the most useful molecular sieves for converting methanol to olefin(s) are silicoaluminophosphate molecular sieves. Silicoaluminophosphate (SAPO) molecular sieves contain a three-dimensional microporous crystalline framework structure of [SiO2], [AlO2] and [PO2] corner sharing tetrahedral units. SAPO synthesis is described in U.S. Pat. No. 4,440,871, which is herein fully incorporated by reference. SAPO molecular sieves are generally synthesized by the hydrothermal crystallization of a reaction mixture of silicon-, aluminum- and phosphorus-sources and at least one templating agent. Synthesis of a SAPO molecular sieve, its formulation into a SAPO catalyst, and its use in converting a hydrocarbon feedstock into olefin(s), particularly where the feedstock is methanol, is shown in U.S. Pat. Nos. 4,499,327, 4,677,242, 4,677,243, 4,873,390, 5,095,163, 5,714,662 and 6,166,282, all of which are herein fully incorporated by reference.
When used in the conversion of methanol to olefins, most molecular sieves, including SAPO molecular sieves, undergo rapid coking and hence require frequent regeneration, typically involving exposure of the catalyst to high temperatures and steaming environments. Moreover, these processes are typically conducted in a fluidized bed reactor where the catalyst is continuously circulated between a reaction zone and a regeneration zone. This continuous circulation necessarily results in collisions between the catalyst composition particles themselves and with the reactor walls which can cause the particles to breakdown into smaller particles called fines. This physical breakdown of catalyst particles is known as attrition and is undesirable because the fines often exit the reactor in the effluent stream resulting not only in catalyst losses but also in problems in downstream recovery systems.
There is therefore a need for a molecular sieve catalyst composition which can be used in the conversion of feedstocks, such as oxygenates, to olefins and which exhibits both a high thermal and hydrothermal stability and a high attrition resistance.
U.S. Pat. No. 6,153,552 discloses that the attrition resistance of a SAPO catalyst can be enhanced by the addition of an external phosphorus source in an amount between 0.1 and 25 wt %, preferably between 1 and 20 wt %, of the finished catalyst. The external phosphorus source is typically a phosphate and is added by mixing with the molecular sieve, an inorganic oxide sol and a clay to form a slurry which is then spray dried. However, the Examples show that, after calcination at 760° C. for 3 hours, the attrition resistance of the catalyst decreases significantly and its surface area falls dramatically.
U.S. Pat. No. 5,110,776 discloses a method of preparing a catalytic cracking catalyst comprising treating a zeolite, such as REY, with an aqueous phosphate solution, combining the resultant aqueous mixture with a matrix precursor, such as alumina, and then spray drying the resultant slurry. The catalyst is reported to have improved attrition resistance and octane when used in catalytic cracking.
U.S. Pat. No. 4,987,110 discloses that an attrition resistant catalytic cracking catalyst can be prepared by spray drying a slurry formed by combining a molecular sieve, which can be a zeolite or a SAPO, with a clay, a silica sol and aluminum chlorohydroxide.
U.S. Patent Application Publication No. 2002/0049133 discloses that an attrition resistant catalytic cracking catalyst can be prepared by forming a slurry of a zeolite having a constraint index of 1 to 12, such as ZSM-5, a phosphorus-containing compound and alumina in an amount less than 10 wt % of the slurry and then spray drying and calcining the slurry.